The field of dilatometry has had a long and unfilled need for a cost efficient dilatometer system capable of providing more accurate CTE measurements. Concerning the more specific field of optical dilatometry, the prior art's emphasis on the use of interferometers that employ collimated or parallel light beams ("parallel light interferometers") has resulted in few advancements in cost efficiency and measurement accuracy.
Parallel light interferometers generate a linear interference pattern, and as used in a dilatometer system, the bright and dark linear fringes of the interference pattern travel across a viewing screen as the temperature of the test sample is varied. To derive CTE, the linear displacement of the fringes with respect to a reference mark on the viewing screen must be measured. Because a reference mark of the viewing screen is used, accuracy of the CTE measurements depends of the stability of the interferometer and the test sample with respect to the reference mark on the viewing screen. To develop a system with maximum stability can be very costly. It would be desirable to develop a system which would not require the use of a reference mark on the viewing screen.
In addition to the stability problems in the prior art, the measurement of linear displacement to derive CTE inherently limits the system's sensitivity and accuracy. For example, it would be desirable to measure a squared function such as area to derive the linear measurements needed to measure CTE, thus substantially increasing accuracy.
Furthermore, in order to maintain accurate fringe detection when using parallel light interferometers, it is also very important to develop a system which maintains parallel light beams. This can be very difficult when the system employs more than one beam splitter or mirror. To maintain parallel light beams requires that the mirrors and beam splitters be aligned within very strict tolerances. To manufacture a system within such tolerances can be very difficult and costly.
The present invention avoids the use of parallel light beam interferometers to resolve the problems described above.
The dilatometry field has also had a long and unfilled need for a system capable of measuring CTE for items of manufacture, e.g., a system capable of measuring CTE of the end product rather than CTE of the material from which it is made. To accomplish this objective requires a system adaptable to annular or circular surfaces so that radial as well as linear CTE can be measured. The apparatus and method of the present invention is capable of measuring both linear and radial CTE and is adaptable to annular as well as flat surfaces.
Accordingly, the present invention provides an optical dilatometer which does not require the use of a reference mark, thereby eliminating the system stability problems discussed above; it permits the measurement of a squared function such as area to determine CTE; it does not require the strict alignment of optical components for maintaining parallel light beams; it is capable of measuring CTE for test samples having either flat or annular surfaces; and it is capable of measuring linear or radial CTE.